1. Field of the Invention
The present invention relates to a physical quantity detection device that detects a physical quantity of the object under detection, and more specifically it relates to a physical quantity detection device which outputs the value of a physical quantity as a digital value. It is suitable for application in, for example, rotational sensors for electric automobiles or other vehicles.
2. Description of the Related Art
In the past, there have been devices developed which control, by means of feedback control, the operating conditions (for example, the rum or position of an object under detection) of a controlled system, such as a vehicle engine, by a variety of sensors (for example, an rpm sensor), a microcomputer being used in these devices to achieve highly accurate control. In these devices, because the analog detection signals obtained from the above-noted sensors cannot be handled as is, the signals from the sensors are captured by means of digitization (encoding).
Examples of devices which detect a physical quantity such as position or rotational speed of the object under detection are those disclosed in the Japanese Unexamined Patent Publication (KOKAI) No. 3-125514 and the Japanese Unexamined Patent Publication (KOKAI) No. 3-220814. The physical quantity detection devices of both No. 3-125514 and No. 3-220814 operate as follows. A pair of oscillating devices each comprising a detection element having characteristics which change in accordance with the change of a physical quantity of an object under detection and an oscillation device, the oscillation frequency of which changes in accordance with change in characteristics of the detection element, is used to generate two pulse signals having a phase difference between them that varies in accordance with the variation in the physical quantity, the phase difference between these pulse signals being encoded by means of a pulse phase difference encoding circuit (a circuit which converts a time difference to a digital value), this phase difference being proportional to the physical quantity that was to be detected. While both of the above-mentioned pulse phase difference encoding circuits have the defects that the encoded value varies with respect to changes in operating conditions such as power supply voltage and ambient temperature, so that the output thereof is different than the true value, the physical quantity detection devices of Japanese Unexamined Patent Publication (KOKAI) Nos. 3-12551 and 3-22081, by detecting the phase difference of the above-noted pulse signals, cancel out the variations caused by changes in operating conditions such as power supply voltage and ambient temperature.
However, because the pulse phase difference encoding circuit used in the physical quantity detection devices of Japanese Unexamined Patent Publication Nos. 3-125514 and 3-220814 obtain a value of the physical quantity by detecting the phase difference between two pulse signals, although it is possible to cancel out variations occurring in the pulse phase difference encoding circuit which are caused by variations in operating conditions such as power supply voltage and ambient temperature for the case in which operating conditions such as power supply voltage and ambient temperature vary slowly with respect to time, a problem existed in that it was not possible to cancel out the variations occurring in the pulse phase difference encoding circuit in the case in which the power supply voltage varies rapidly with respect to time.
The time A/D conversion circuit (pulse phase difference encoding circuit) shown in FIG. 7 was proposed, as disclosed by the inventors of the present invention in Japanese Unexamined Patent Publication No. 5-37378, for the purpose of suppressing errors caused by rapid changes in the power supply voltage with respect to time.
This time A/D conversion circuit, which is a circuit which converts a time difference as noted above to a digital value, operates as follows. First, the start signal PA is input, after which the signal to be timed PB is input. Additionally, time reference signal PC is input, this signal being generated by a crystal oscillator and having a stable frequency which is not influenced by temperature or power supply voltage variations. As shown in the drawing, the time difference between PB and PA is detected by a pulse phase difference encoding circuit 108a, and the circuit 108a outputs signal D.sub.AB, and the time difference between PC and PA is detected by the pulse phase difference encoding circuit 108b, and the circuit 108b outputs signal D.sub.AC. D.sub.AB is then divided by D.sub.AC to determine the signal time D.sub.AB with respect to the reference time D.sub.AC, thereby outputting a stable measured time that is not influenced by variations in temperature and power supply voltage.
However, in the above-noted time A/D conversion circuit shown in Japanese Unexamined Patent Publication (KOKAI) NO. 5-37378, even though a signal generated by a crystal oscillator is used as time reference and the time is measured with high accuracy, it is very difficult to detect a physical quantity of the object to be measured.